Certain tetraalkyl esters of {[(o-phenylene)bis[imino(thiocarbonyl)]}diphosphoramidic acid are disclosed in a Japanese patent application, the abstract of which has appeared in Chemical Abstracts at Volume 82, page 107520g. The application, Japanese No. 007108, filed 01/17/73 was published Sept. 5, 1974, No. 74-93541, filed by Ube Industries, Ltd., assigned to Institute of Physical and Chemical Research. The disclosed esters are said to be effective fungicidal agents. However, the claimed compounds per se of the present invention are not described therein; nor is it anticipated or suggested that the above-said compounds would be effective for the control of helminths in warm-blooded animals. Phosphonothioureides are also disclosed in U.S. Pat. Nos. 4,076,809 and 4,086,336. These disclosures, however, do not claim or disclose para-phenylene bis[imino(thiocarbonyl)] diphosphoramidates esters as anthelmintic agents. Furthermore, it is indicated in U.S. Pat. No. 4,086,336 that the preferred compounds are ortho-phenylene compounds. Several of these phosphonoureides are found to be toxic and less effective than the para-phenylene derivatives in a series of anthelmintic assays.